The present invention relates generally to a navigation system for an automobile or any motor vehicle. More particularly the present invention is concerned with a navigation system adapted to be installed on a motor vehicle for detecting a current position thereof to display it on a road map produced on a display screen together with a destination of the motor vehicle as well as the routes which lead to that destination.
In general, the navigation system for the motor vehicle is designed to display the current position of the motor vehicle, or car, on a guidance road map generated on a display unit for aiding a driver in driving his or her car in unfamiliar districts and/or in the nighttime.
One typical example of the navigation systems for the motor vehicle known heretofore is disclosed in Japanese Patent Laid-Open No. H1-119898 (JP-A-H1-119898). This known navigation system is shown in a block diagram in FIG. 9. Referring to the figure, the known navigation system includes an information/command input unit 1 (hereinafter also referred to as a manipulating unit) for inputting information concerning a range within which a motor vehicle equipped with the navigation system is planned to travel as well as other information and commands. Operatively connected to an output of the manipulating unit 1 is a search range limiting unit 2 which serves for arithmetically determining a linear distance between the current location or position of the motor vehicle and the destination as inputted, to thereby limit or constrict the range within which routes to be followed by the motor car are searched. A map memory 3 for storing information concerning a road network and road segments which constitute the road network is provided in association with a time memory 4 which stores information concerning the times required for driving the motor vehicle along the road segments stored in the map memory 3, the required times being classified according to months, days of the week and time zones. Connected to the output ports of the map memory 3 and the time memory 4 is a route search unit 5 which serves to search all possible combinations of the road segments (hereinafter referred to as the route sets) which can be selected for the motor vehicle to reach the destination from the current position on the basis of the road network information stored in the map memory 3 and the times required for driving the motor vehicle along the routes as selected on the basis of the time information stored in the time memory 4. The output information from the route search unit 5 is supplied to a route extracting unit 6 for selectively extracting the route set which requires the shortest time for the motor vehicle to reach the destination on the basis of the required times involved in driving the motor vehicle along the route sets searched by the route search unit 5. The route set as extracted will hereinafter be referred to as the temporally shortest route. Further, a current vehicle position detecting unit 7 is provided for arithmetically determining the azimuth direction and the distance covered by the motor vehicle to thereby estimate the current position of the motor vehicle. Connected to the outputs of the route extracting unit 6 and the current vehicle position detecting unit 7 is an emphasis signal generator 8 for generating an emphasis signal for displaying the temporally shortest route extracted through the route extracting unit 6 and the current vehicle position with emphasis on the road network map. The output of the emphasis signal generating unit 8 is supplied to a guidance unit 9 for guiding the motor vehicle with the aid of the emphasis signal supplied from the emphasis signal generating unit 8. The required times taken actually for driving the motor vehicle along the road segments as extracted are stored in the time memory 4 by the guidance unit 9. Finally, reference numeral 10 denotes a display unit which is supplied with the aforementioned emphasis signal for displaying the temporally shortest route together with the current vehicle position with emphasis.
Now, referring to a flow chart shown in FIG. 10, description will be made of the temporally shortest route generating operation and the vehicle guidance operation of the known navigation system having the structure described above.
In a step S1, decision is made by the search range limiting unit 2 as to whether or not initialization input information such as that of the destination is loaded through the manipulating unit 1. If the initialization information is loaded (i.e., when the answer of the step S1 is "YES"), the processing proceeds to a step S2, where the route search unit 5 reads out the road segment information from the map memory 3 and then fetches from the time memory 4 the data of required times involved in driving the motor vehicle along the road segments which are read out from the map memory 3 in a step S3, to thereby search the route sets which can lead the motor vehicle to the destination by combining together the road segments in a step S4. Subsequently, in a step S5, the route extracting unit 6 arithmetically determines the times required for driving the motor vehicle along the individual road segments sequentially for each of the route sets as searched. Further, in a step S6, the temporally shortest route (i.e., the route allowing the motor vehicle to reach the destination within the shortest time) is selected. The temporally shortest route thus determined is stored in the time memory 4 in a step S7 on one hand and displayed on the display unit 10 in a step S8 on the other hand. In a step S9, decision is made as to whether or not the motor vehicle has reached the destination. If not (i.e., if the answer of the decision step S9 is negative or "NO"), the processing of the step S9 is repeated, while otherwise the processing proceeds to a step S10 in which the times actually taken for driving the motor vehicle along the individual road segments are stored in the time memory 4. Finally, in a step Sll, the temporally shortest route information previously stored in the memory 4 is canceled, whereupon a return is made to the step S1.
In contrast, if there is no input of the initialization information (i.e., the answer in step S1 is "NO"), the processing proceeds to a step S12, where a road network for the planned driving or in which the motor vehicle is currently running is displayed on the display unit 10. In a step S13, the required times actually taken for driving the motor vehicle along the individual road segments are stored in the time memory 4, whereupon the step S1 is resumed.
As will be understood from the above description, in the known navigation system, the driver first designates the destination through the manipulating unit 1. In response., the search range limiting unit 2 properly limits or narrows the route search range on the basis of a straight line connecting between the current position of the motor vehicle and the destination. Subsequently, the route search unit 5 searches all the possible routes leading to the destination within the limited range on the basis of the road network information stored in the map memory 3, while the route extracting unit 6 calculates the times required to reach the destination for all the routes as searched on the basis of the required time information stored in the time memory 4 to compare the required times for all the searched routes with one another to thereby select the temporally shortest route, whereon the emphasis signal generating unit 8 generates an emphasis signal for displaying the temporally shortest route as well as the current vehicle position. The guidance unit 9 responds to a guidance start signal to display on the display unit 10 the temporally shortest time route together with the current vehicle position of the motor vehicle with emphasis. In this manner, visible information is made available for aiding the driver in driving his or her motor vehicle along the temporally shortest route.
The conventional navigation system installed on the motor vehicles such as described above suffers from numerous problems mentioned below. First, with the navigation system known heretofore, there often arise such situations in which a roundabout or bypass road desired by the driver is not selected as a part of the temporally shortest route. A road segment of a high traffic snarl or a high traffic congestion frequency can possibly be selected. For example, the frequency of traffic congestion is defined as the number of occurrences of congestion per day. Further, when the current position of the motor vehicle is deviated from the guidance route, troublesome operation is required for determining again the guidance route. Besides, the guidance route determined in the past can no more represent the temporally shortest route when a predetermined time has lapsed during parking on the way of driving the motor vehicle along the guidance route. Moreover, the arithmetic determination of the temporally shortest route can not be performed before the start of driving even when the starting time is set beforehand, as a result of which the start of the motor vehicle has to be correspondingly delayed until the arithmetic operation activated in response to the operation of a route guidance start key has been completed. Furthermore, in case where there are a plurality of side destinations (i.e. places at which the motor vehicle is to drop in) on the way to the final destination, the sequence of the side destinations has to be decided by the driver himself or herself since the navigation system can set only one destination at a time.